Profiling in Haskell for a 10x Speedup

May 19, 2019

I wrote up a toy project in Haskell and profiled it to learn about Haskell’s profiling tools and about profiling code in general. Profiling in Haskell with Stack is super easy; to prove it I’ll walk through the problem I was trying to solve, my slow first solution, and how I used Haskell’s profiling tools to find and fix some egregiously slow parts of the code.

I had three takeaways from this little project:

The source code and profiling data for this project is all available on GitHub. Also the Appendix below has a bunch of links to help you find the interesting parts of the code.

Problem: simulating probabilities

The problem I wanted to solve was to simulate the probability of winning one carnival game I got to play while on vacation recently. The game itself is super simple and purely luck-based. The rules:

  1. There’s a 6 × 6 grid, each with a special character identifying it.
  2. There are 36 tiles, each with a character matching one grid space (and there are no duplicates, so all characters are accounted for).
  3. Initially, all tiles are placed face down.
  4. To play, a contestant chooses 15 of the 36 tiles and flips them over.
  5. The contestant then places the flipped tiles onto the correct spots.
  6. If placing the 15 tiles forms a bingo in any row, column, or full diagonal, it’s a win. Otherwise, it’s a loss.

The game setup in real life looked something like this:

Taiwan Carnival Bingo

(Image credit)

My question was: how lucky should we considider ourselves if we win? I’m sure I could have answered this exactly with some combinatorics, but that seemed boring. Instead, I wanted to write a program solve it: generate random boards, and check how many of them have bingos.

Naive solution

In the course of playing around with this problem, I implemented a bunch of different solutions—about 5 in total, each one faster than the last. At a high level, each solution followed this pattern:

All the solutions exploited the fact that we don’t care where the characters on the board are nor what characters are on the tiles. The only thing that matters is whether a tile ends up on a specific grid space, which means boards can just be vectors of bits.

Attempt #1 was really slow, so we won’t talk about it 😅

Attempt #2 was a little bit faster (but not by much, which made me disappointed—more on this later). The solution looked like this:

shuffleBits :: RandomGen g => g -> Board -> Int -> (Board, g)
shuffleBits gen board 1 = (board, gen)
shuffleBits gen (Board bs) n =
  -- (Maybe) swap the current MSB with one of the lesser bits
  let (i, gen') = randomR (0, n - 1) gen
      bs'       = swapBits bs (n - 1) i
  -- Recurse on the lesser bits
  in  shuffleBits gen' (Board bs') (n - 1)

Clever, but not in the right ways

All told, I thought the approach in Attempt #2 was pretty clever. It used a single Word64 (instead of a larger structure like a list) to represent the board, so it shouldn’t have had needed many allocations. And because it was just a Word64, it could use bit operations to manipulate the board and check for bingos, avoiding the need to walk a large structure.

But when I ran this on my 2017 MacBook Pro (i7-7920HQ CPU @ 3.10GHz, 16 GB memory), it was still really slow:

❯ time .stack-work/install/x86_64-osx/lts-13.21/8.6.5/bin/bingo-sim 100000
Trials:   100000
Bingos:   3529
Hit rate: 0.03529
  0.71s user  0.02s system  98% cpu  0.738 total

(Note that we’re running time on the compiled binary directly, instead of running with stack exec --; we don’t care for the overhead from running via Stack).

So even after using a bit vector for the board, it took 738ms. To be clear, these results for Attempt #2 were an improvement over my even slower Attempt #1, but not by much—maybe by 200ms. 700ms+ seemed excessive.

How to profile Haskell code

Determined to make it faster, at this point I resigned myself from guessing and looked up how to profile Haskell code. Turns out, with Stack it’s super simple:

# Rebuild with profiling information
❯ stack build --profile

# Run the code with runtime profiling enabled
❯ stack exec -- bingo-sim 100000 +RTS -p -RTS

That’s all it took, and it generated a file that had a bunch of lines that looked like this:

COST CENTRE              MODULE  SRC  no.  entries  %time %alloc   %time %alloc
  randomR                ...     ...  ...  ...       0.8    0.0    84.9   83.5
  randomIvalIntegral     ...     ...  ...  ...       6.9   13.4    84.1   83.5
   randomIvalInteger     ...     ...  ...  ...      21.3   29.2    77.2   70.1
    randomIvalInteger.f  ...     ...  ...  ...      11.0    2.4    41.5   31.5

There’s a bunch of extra information in the actual .prof file (which you can see on GitHub) but the important parts for me were the four columns at the end. The first two are the proporion of execution time and allocated memory attributable to this cost center specifically. The last two are the same, but summed over all child cost centers.

(I found the GHC User Guide super helpful to learn everything I wanted to know: what the columns mean, what a cost center is, and some tips for profiling in general).

So the glaring realization in the data: we’re spending 85% of our 738ms running time generating random numbers. All my effort spent optimizing memory, but it was the PRNG that was slow the whole time. 😣

Fast PRNG in Haskell

738ms × 0.849 ≈ 600ms+ spent generating random numbers seemed like an absurd abount of time. Non-cryptographically secure pseudo random number generation shouldn’t take this long. And indeed, after a bit of searching I came across this reddit thread complaining about how slow System.Random in Haskell is for PRNG, and then moments later I found this article from Chris Wellons comparing various PRNGs for performance… Exactly what I was looking for 👌

So for Attempt #3, I took his suggested PRNG and ported it from C to Haskell.

And lo and behold, it was faster. A lot faster:

❯ time .stack-work/install/x86_64-osx/lts-13.21/8.6.5/bin/bingo-sim 100000
Trials:   100000
Bingos:   3670
Hit rate: 0.0367
  0.11s user  0.01s system  90% cpu  0.126 total

Yep. That just went from 738ms to 126ms, for a 5.8x speedup 🤯

With a result this good, we might ask ourselves what we had to give up in the process—things this good usually come at a price. The biggest one that I notice is that the API I provide for random number generation is far less generic.

My Prng.hs is a direct translation of the C to Haskell. It only generates 64-bit unsigned ints. System.Random has an arguably nicer API, using type classes to generate random ints, characters, etc., allowing users to implement generators for their custom types, and having helpers for generating random values within a range and sequences of random numbers.

It’s possible we could prune some of the fat from System.Random’s default implementations (without also changing the underlying random number generator) and see a considerable speedup. It’s also possible we could make Prng.hs export instances of the appropriate type classes, and again see a speedup.

But considering that I wasn’t using any of that extra stuff, I figured I’d just keep it simple. The code to generate random boards hardly changed:

shuffleBits :: Prng.State -> Board -> Int -> (Board, Prng.State)
shuffleBits gen board 1 = (board, gen)
shuffleBits gen (Board bs) n =
  let n'           = n - 1
      (rand, gen') = next gen
      -- Uses `mod` instead of `randomR` to generate within a range
      i            = rand `mod` (fromIntegral n)
      bs'          = swapBits bs n' (fromIntegral i)
  in  shuffleBits gen' (Board bs') n'

Further speedups

Spurred on by the thrill of the previous speedup, I kept going. By this time I’d learned the value in following the .prof output. The output led me to Attempt #4, which refactored the PRNG into CPS avoid allocating a tuple, and then Attempt #5, where I added some BangPatterns.

At the end of it all, my simulation ran in just 70ms!

❯ time .stack-work/install/x86_64-osx/lts-13.21/8.6.5/bin/bingo-sim 100000
Trials:   100000
Bingos:   3670
Hit rate: 0.0367
  0.06s user   0.01s system   92% cpu   0.070 total

This is a 10x speedup over my dissapointing Attempt #2, and a 12.8x speedup over my super naive Attempt #1. Not bad for a first attempt at profiling in Haskell!

I found this super encouraging. Given how easy the tooling is to get started with, how well documented things are, and my satisfaction with the results, I’m very likely to reach for profiling tools in the future.

Appendix: Links

The entire source is on GitHub.

I wrote up all five of my attepts as separate GitHub commits, so if you want to compare and constrast the approaches feel free:

I also had never used Haddock before, so I used this project as an opportunity to learn how to build and write Haddock. I published the docs on GitHub pages if you want to browse them:

And I also put the source for all the .prof files I generated for each attempt:

If you have any questions about anything, feel free to reach out!

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